The present invention relates to the formation of semiconductor devices. More particularly, the present invention relates to the formation of an oxide layer as part of a device or as used in the fabrication of the device.
In the semiconductor industry, oxide films are used in a variety of applications. Oftentimes they are used for scratch protection and passivation purposes. Oxide films are also used as a dielectric or insulative layer, electrically separating various regions or structures. For example, an oxide film can be used as a dielectric between different levels of metal in a semiconductor device. Such a film could also be used for field isolation. Moreover, an oxide film can serve as a gate oxide, wherein the film is provided above an area, such as a semiconductor substrate, having a source region, a drain region, and an interposing channel region. A gate, in turn, is formed on the oxide film. As a result, the voltage applied to the gate must reach a particular threshold before overcoming the insulative effects of the oxide and allowing current to flow through the channel. When used as field isolation, an oxide is formed in order to electrically insulate one device, such as a transistor, from another.
Whether for field isolation purposes or for application in the gate stack of a transistor, providing the oxide typically begins by exposing designated oxide regions of a substrate to an oxidizing ambient through a patterned mask. The mask may be made, for example, of silicon nitride. For purposes of explaining the current invention, it is assumed that the substrate represents the surface of a wafer and is comprised generally of silicon. Nevertheless, this invention is understood to cover devices having a substrate comprising any construction made of semiconductive material, including but not limited to bulk semiconductive materials such as a semiconductor wafer (either alone or in assemblies comprising other materials thereon) and semiconductive material layers (either alone or in assemblies comprising other materials). Upon exposure to the oxidizing ambient, the unprotected portions of the silicon substrate oxidize into silicon dioxide (SiO2). The silicon at and below the surface of the substrate that oxidizes is often referred to as having been xe2x80x9cconsumed.xe2x80x9d It follows that the amount of silicon consumed can indicate the depth of SiO2 beneath the substrate""s original surface. As a result, greater consumption allows for a greater depth of SiO2 and, thus, greater electrical isolation between devices or between active areas within a device.
The consuming effect of oxide films on silicon serves other purposes as well. For example, greater consumption in a particular area of the wafer allows access to a lower level of silicon within the substrate. Accordingly, removing the oxide results in a wafer topography having different elevations of silicon, depending upon the amount of prior oxidation in each area. This is particularly helpful in embedded dynamic random access memory (DRAM) processing, wherein the memory cell array should be embedded deeper within the wafer than other memory elements.
Oxidizing the exposed substrate, as discussed above, is often referred to as xe2x80x9cgrowingxe2x80x9d the oxide. Oxides can be grown in a xe2x80x9cdryxe2x80x9d process using oxygen (O2) or in a xe2x80x9cwetxe2x80x9d process using steam as the oxidizing agent. As an alternative to growing, oxides can be deposited on the substrate with techniques such as sputter deposition or chemical vapor deposition (CVD).
Oxide layers have a large impact on device performance due to their role in isolating active device regions and in establishing voltage thresholds for devices. Thus, there is always a need in the art for high quality oxide films. Further, as the dimensions of semiconductor devices are scaled down to enhance circuit density and speed, the oxide films must advance accordingly. Therefore, those skilled in the art are constantly striving to provide oxide films that are thinner and that have a high dielectric constant.
However, during the deposition or growth of oxides, defects in the oxide can occur due to the presence of certain constituents within the layer, such as contaminants exposed to the oxide. For example, particulate matter in the process atmosphere is one source of contamination. Even when the oxide or other layers are developed in a xe2x80x9cclean roomxe2x80x9d environment, wherein filters and other techniques attempt to remove particles from the environment, particles that are too small for these techniques to handle may nevertheless end up within the oxide layer. Further attempts at reducing defects have been made by clustering together the chambers for several wafer processes in an environment isolated from and even more controllable than the clean room atmosphere. Transferring the wafers between the clustered chambers can involve the use of a wafer carrier capable of maintaining a vacuum or a nitrogen atmosphere. See, for example, U.S. Pat. Nos. 5,613,821 and 5,344,365. Nonetheless, there is a constant need in the art for further lowering the number of defects in oxide films, including a need for methods of handling contaminants that find their way to the wafer despite the controlled environment.
Accordingly the current invention concerns methods for providing an oxide layer during the processing of a semiconductor device. One exemplary embodiment relates to a method wherein an oxide is provided on a substrate surface and is then subjected to a cleaning process, followed by a provision of still more oxide. The oxide in either step could be grown or deposited. Moreover, the cleaning step may be used to remove all or some of the first provision of oxide. This embodiment has the advantage of removing any oxide that may carry constituents such as contaminants that were part of the underlying substrate. Thus, this embodiment can be used to provide a more contaminant-free oxide for a semiconductor device. Alternatively, this embodiment can be used to selectively consume portions of a substrate, thereby allowing memory structures such as embedded memories to be formed within the lower elevations of the substrate.
Another exemplary embodiment allows for providing a gate dielectric having a high dielectric constant. Such dielectrics include oxides such as tantalum pentoxide (Ta2O5), or layers produced through rapid thermal nitridation (RTN), such as oxynitrides. In this embodiment, a layer of oxide or oxynitride serves as an adhesion layer between the substrate and the subsequently deposited Ta2O5. A cleaning step between providing the adhesion layer and providing the Ta2O5 layer is optional. One advantage of this embodiment is that leakage current can be improved.
Yet another exemplary embodiment covers a range of steps for processing the semiconductor device, including a vapor clean, an initial oxide growth or deposition, a subsequent oxide growth or deposition, an optional second vapor clean between the two oxide steps, an oxide hardening, and the formation of an electrode over the second oxide. In a more preferred version of this embodiment, these steps are clustered, wherein transportation between the various processes are performed in a common controlled environment, such as a nitrogen atmosphere or a vacuum. The cluster process environment lowers the amount of contaminants having access to the in-process semiconductor device, and the cleaning steps help to negate the effects of any contaminants that appear within the device despite the attempts to control the environment.